1. Field of the Invention
This invention relates to a liquid developer apparatus and method for bringing a liquid developer into contact with a recording medium having an electrostatic latent image, and more particularly to a liquid developer apparatus and method with a function for removing surplus toner in a liquid developer and cleaning the apparatus.
2. Description of the Related Art
A liquid developer apparatus for forming an electrostatic latent image on a recording sheet such as a recording paper sheet, and developing the image on the recording sheet with the use of a liquid developer containing toner has conventionally been used as a recording apparatus for recording an image on a recording sheet of a large size such as "A0" size.
Such a conventional liquid developer apparatus is disclosed, for example, in Japanese Patent Application KOKAI Publication No. 1-185569. This apparatus has a single electrostatic recording head and a plurality of liquid developer units for liquid developers of different colors. In the apparatus, a color image is recorded on a recording paper sheet by reciprocating the sheet.
In general, to develop, by a developer apparatus, an electrostatic latent image of a minus potential recorded on a recording medium, a liquid developer containing a toner charged with plus electricity is used. In this case, application of a minus voltage to the developer unit prevents movement of plus electricity-charged toner particles toward an image background portion of the recording medium, thereby preventing occurrence of fog in the background portion.
However, there is a case where the liquid developer contains toner particles charged with electricity of an opposite polarity (i.e. a minus potential) because of insufficient adjustment of electric characteristics. Furthermore, the liquid developer may gradually deteriorate with the passing of time, resulting in an increase in the amount of toner particles charged with electricity of the opposite polarity. Thus, if a minus voltage is applied to the developer unit which receives a liquid developer containing both toner particles charged with electricity of a predetermined polarity and those charged with electricity of a polarity opposite to the former, the toner particles charged with electricity of the opposite polarity, i.e. charged with minus electricity, repel the developer unit charged with minus electricity, and stick to the electrostatic recording paper sheet, thereby increasing the degree of fog in the image background portion.
Moreover, there is a developer unit with a developer groove formed in its upper surface, which groove serves as a passage of a liquid developer when a recording paper sheet has been placed on the developer unit to close the groove. In this developer unit, that portion of the upper surface of the unit which contacts the recording paper sheet serves as a developer electrode. Toner particles between the upper surface and the recording paper sheet are attracted by the recording paper sheet and adhered thereto when an electrostatic latent image has been formed thereon. Therefore, when that portion of the recording paper sheet which has a high potential has passed a corresponding upper surface portion of the developer unit, toner particles on the corresponding upper surface portion are completely removed, and the corresponding upper surface portion has its metal ground exposed (in other words, the upper surface portion is cleaned).
On the other hand, toner particles remain on that upper surface portion of the developer unit on which the high potential portion of the recording paper sheet has not passed. If in this state, the developer unit is moved downward and separated from the recording paper sheet, the toner particles remain on the unit, and stick thereto after the solvent of the developer evaporates. Where toner particles cover part of the upper surface of the developer unit, the characteristics of the developer unit as a developer electrode are degraded, and an image formed on that portion of the recording paper sheet which corresponds to the toner-covered portion of the developer unit will inevitably have a low toner concentration.
If a different image is recorded on a new recording paper sheet by means of the above-described partially toner-coated developer unit, that portion of the image which corresponds to the completely toner-removed portion of the developer unit has a high toner concentration, and the other portion of the image has a low toner concentration. This means that the newly recorded image is influenced by the previously recorded image.